The seasonal burden of influenza, coupled with the with the recent pandemic outbreaks of more pathogenic strains such as the 2009 and 2013 H1N1, underscores the critical need for understanding the pathophysiology of influenza injury. A hallmark of severe influenza infection is thymic atrophy and associated peripheral lymphopenia. Given that lymphopenia is strongly associated with secondary bacterial pneumonia, Adult Respiratory Distress Syndrome (ARDS) and mortality, understanding and targeting ways to promote thymic regeneration and reduce lymphopenia can have great impact on patient care. Our laboratory is focused on the balance between the cytokine interleukin-22 (IL-22) and its soluble receptor IL-22Ra2 (IL-22 binding protein or IL-22BP). The central hypothesis for this application is that IL-22/IL-22BP balance is required for thymic homeostasis and loss of this balance during influenza infection allows for thymic atrophy. We hypothesize that IL-22 is integral in thymic maintenance and during influenza infection there is an increase in IL-22BP, preventing IL-22 activity. Our rationale is that constitutive expression of IL-22 is integral for thymic maintenance as IL-22-/- mice undergo extensive thymic involution by 6-8 months. During influenza infection, we find there is no change in IL22 message, but a significant decrease in IL-22 protein. This coincides with a significant (~100fold) increase in IL-22BP. Moreover, in highly novel preliminary data, we have found IL22BP-/- mice, which have uninhibited IL-22 activity, are protected from lethal influenza infection, do not undergo thymic atrophy and have greater numbers of CD4/CD8 double positive cells than wild type mice. Thus, we believe the IL-22/IL-22BP balance can be exploited therapeutically to reduce thymic loss and lymphopenia during experimental influenza infection. A major barrier to investigating IL-22BP is a lack of reliable reagents. As such, in this R21 we will develop a novel IL-22BP reporter mouse as well as test our IL-22BP neutralizing antibodies. We will use these reagents to test our hypothesis in two specific aims. In the first aim we will test the requirement of IL-22 in reducing atrophy and promoting thymic regeneration after infection. We will identify the cells that produce IL-22, and the cells IL-22 interacts with in the thymus during influenza infection. We will determine the therapeutic effects of IL-22 by administering IL-22:Fc at different times after infection, and we will determine if the effects of IL-22 are intrinsic to the thymus by using IL-22Ra1fl/fl mice crossed to global (EIIa) or thymus epithelial specifc (Foxn1) cre lines. In the second aim we will test the hypothesis that IL-22BP inhibits IL-22 activity allowing atrophy and lymphopenia to occur. We will identify the cells that produce IL- 22BP. We will use a reductive approach (IL-22BP-/- mice) to study the importance of IL-22BP during infection and finally we will use a neutralizing antibody to determine the therapeutic potential of targeting IL-22BP during influenza infection. We believe that by targeting IL-22BP or giving IL-22, we can reduce the impact of influenza on the thymus and subsequent peripheral lymphopenia allowing for improved patient outcomes.